Titanium dioxide is used widely as a white pigment and the like. Titanium dioxide includes two crystallographic systems, i.e., anatase-type one which is of a low-temperature stable phase and rutile-type one which is of a high-temperature stable phase. When used as a pigment, there is made a good use of them according to their characteristics. For example, anatase-type titanium dioxide is characterized in that it is bluish in color tone as compared with the rutile-type one.
Hitherto, anatase-type titanium dioxide has been manufactured by a sulfuric acid method on an industrial scale. This manufacturing method is usually performed by hydrolyzing an aqueous titanyl sulfate solution to produce hydrous titanium dioxide slurry, calcinating the slurry at 850 to 1100.degree. C. to obtain anatase-type titanium dioxide powder having a predetermined particle size.
Such conventional anatase-type titanium dioxide has a problem in that as compared with rutile-type titanium dioxide, it tends to be discolored so that it has low light resistance and low weatherability. More specifically, generally titanium dioxide crystals have more or less partial structural defects and an increase in the structural defects reduces their chemical stability so that when they are used as a pigment, they tend to be discolored due to external energy such as ultraviolet rays, heat, grinding force or the like. The anatase-type titanium dioxide manufactured by the conventional sulfuric acid method has more crystal defects and hence tends to be discolored than rutile-type titanium dioxide. In particular, when titanium dioxide is used as a colorant for plastics, it is a recent trend to use higher kneading temperatures; sometimes the treatment temperature may exceed 300.degree. C. The conventional anatase-type titanium dioxide suffers considerable discoloration at treatment temperatures of 300.degree. C. or higher, thus deteriorating the color tone of the plastics.
The present invention is to solve the above-mentioned problems associated with the conventional anatase-type titanium dioxide and has for its object to provide anatase-type titanium dioxide which has a high degree of whiteness, being difficult to discolor by high-temperature treatments, and has good chemical stability with excellent light resistance and weatherability and to provide a manufacturing method therefor. In the following explanation, sometimes, the state of having a high degree of whiteness, being difficult to discolor by high temperature treatments, and having good chemical stability with excellent light resistance and weatherability is referred to as having high color stability for convenience's sake.